Another binocular comet? You better believe it. Comet Johnson takes center stage at nightfall this month and next.
Nothing against Giacobini, Kresak, Mrkos, and Pajdusakova, but this is one comet name I can pronounce with confidence. Even better, it's been humming along very well, thank you, while waiting for its turn at center stage.
At magnitude +8.5, Comet Johnson (C/2015 V2) is already bright enough to join the ranks of this year's band of binocular comets: NEOWISE (C/2016 U1), 45P/Honda-Mrkos-Pajdusakova, 2P/Encke, 41P/Tuttle-Giacobini-Kresak, Lovejoy (C/2017 E4), and PanSTARRS (C/2015 ER61). Comet watchers appreciate the bonanza; we've been happily toting out scopes and binoculars to follow the progress of each in its turn.
As the Moon toddles east and wanes, dark skies return as soon as May 12th. The timing couldn't be better, with Comet Johnson making a steep dive through the constellation of Boötes high in the southeastern sky at nightfall while also reaching peak brightness.
I last caught sight of the comet shortly just before dawn on May 6th. In 10×50 binoculars, Johnson was a faint, patchy glow in Canes Venatici. The view in my 15-inch reflector was more satisfying. At 76×, Johnson displayed a moderately condensed coma about 8′ across with a ¾° long broad, diffuse tail pointing northwest. Upping the magnification to 286×, I could see a tiny, almost stellar nucleus of magnitude +13.5 at coma center.
Studying a comet's nucleus is a strange experience. At low magnification, it might appear fairly bright, but the more you magnify, the smaller and fainter the nucleus (pseudo-nucleus actually, since the true nucleus is hidden by reflective dust) becomes until you're staring at just a faint pinprick of light at the heart of a dusty maelstrom.
Comet Johnson currently displays the two classic types of tails: a broad, brighter dust tail and at nearly a right angle to it, a narrow ion or gas tail. The dust tail, while quite diffuse, was easy to see. I suspect even a 6-inch will show it as an extension of the brighter coma to the northwest. I had no luck, however, discerning the ion tail. Let's hope that changes in the next few weeks as the comet continues to brighten, perhaps reaching magnitude +8 at peak in early June.
Comet Johnson (C/2015 V2) was discovered by J. A. Johnson on November 3, 2015, on CCD images taken with the Catalina Sky Survey's 0.68-m Schmidt telescope. It passes closest to the Earth on June 5th at a distance of about 120 million km (75 million miles) and reaches perihelion a week later, on June 12th.
For the moment, northern hemisphere skywatchers have the best view, but by early June, everyone will get a piece of the action. The comet plunges south throughout the early summer, crossing into Virgo in mid-June and Centaurus by the end of July.
You may be wondering how all those other comets we've been tracking this spring are faring. Lovejoy (C/2017 E4), which experienced a bright outburst in March and became an easy binocular object in early April, has literally lost its head, becoming little more than a tail streak. It's now fainter than magnitude +12! PanSTARRS (C/2015 ER61) is a temptingly-bright magnitude +7.5 with a well-condensed coma about 6–8′ across and a ½° tail pointing west-southwest.
While best viewed from the tropics, you can still catch it from mid-northern latitudes. You'll need to rise early and observe from a site with a wide-open view of the eastern horizon, however. The comet only climbs to an altitude of 6-8° before the start of morning twilight. I've included a map to get you there.
Comet 41P/Tuttle-Giacobini-Kresak remains a diffuse, round glow but has now shrunk to about 10′ in diameter and faded to magnitude +9. Watch for it to slowly slide from Lyra into eastern Hercules during the remainder of May.
Unless a new, bright comet is discovered, Johnson will be our last binocular-bright comet of the year. Catch it if you can!